Lead and zinc sulphides generally undergo similar oxidation-reduction reactions. As a result, there is no known method to leach and recover zinc selectively from composite lead-zinc sulphidic minerals. This invention deals with a selective leaching and recovery of zinc from composite zinc and usually lead-bearing sulphides, which are either in the form of complex zinc and lead metal containing sulphidic minerals, or in the form of zinc sulphide concentrates, in-situ- or ex-situ in an economic and environmentally friendly manner.
Zinc is the fourth most common metal in use, trailing only iron, aluminium, and copper. It is normally found in association with other base metals such as copper and lead in naturally occurring ores. Zinc has a low affinity for oxides and prefers to bond with sulphides. Sphalerite, which is a form of zinc sulphide, is the most heavily mined zinc-containing ore. The major uses of zinc are anti-corrosion coatings on steel (galvanizing), precision components (die casting), construction material, brass, dry batteries, pharmaceuticals and cosmetics and micronutrient for humans, animals and plants. The oxide is used in the manufacture of paints, rubber products, floor coverings, plastics, printing inks, soap, textiles, electrical equipment, and other products.
Conventional extractive metallurgical process generally involves pyrometallurgical methods for recovering zinc values from zinc sulphides. Known recovery process mostly involves grinding the ore, froth flotation (which selectively separates minerals from gangue by taking advantage of differences in hydrophobicity) to get an ore concentrate, roasting and reduction with carbon or electrowinning. However, such treatment often entails expensive mining and beneficiation process steps to concentrate the sulphides. In addition, the production of zinc employing the known technology from sulphidic zinc ores produces large amounts of sulfur dioxide, carbon dioxide and cadmium vapor. Smelter slag and other residues of process also contain significant amounts of heavy metals. The dumps of the past mining operations leach significant amounts of zinc and cadmium. Soils contaminated with zinc through the mining of zinc-containing ores, refining, or where zinc-containing sludge is used as fertilizer, can contain several grams of zinc per kilogram of dry soil. Levels of zinc in excess of 500 ppm in soil are deemed to interfere with the ability of plants to absorb other essential metals, such as iron and manganese. Further, strict adherence to environmental regulations governing mining operations may substantially increase the cost of recovering zinc from its ores by conventional processes.
A patent search revealed only approaches to simultaneously leach both lead and zinc from composite lead-zinc sulphidic minerals. Geisler in U.S. Pat. No. 5,523,066 and Turner in U.S. Pat. No. 6,726,828, describe use of in-situ leach mining utilizing a mixture of acetic acid and hydrogen peroxide (for sulphide oxidation) to recover Ca, Mn, Pb and Zn as a combined leachate from a permeable geological host. Both methods employ hydrogen peroxide as an oxidant. The decomposition of hydrogen peroxide with time and its effect on the overall recovery process is left unexplained. U.S. Pat. No. 4,500,398 uses fluosilicic acid with an oxidant to dissolve sulphides. Neither of these methods suggests selective leaching of zinc from composite lead-zinc sulphidic minerals proposed herein.